Age- and Sex-dependent Isozyme Expression in a Wisconsin Population of Adult Alfalfa Weevils, Hypera postica (Coleoptera: Curculionidae)

Abstract

A study of isozyme variability within a population of adult alfalfa weevils, Hypera postica (Gyllenhal), revealed that the isozyme expression of glucose 6-phosphate dehydrogenase (G6PDH), aldehyde oxidase (AO), and lactate dehydrogenase (LDH), 3 of the 15 enzymes examined, appeared to vary with time (season) of collection. Polyacrylamide gel electrophoresis was used to compare the expression of G6PDH, AO, and LDH in newly eclosed adult weevils with expression of the three enzymes in overwintered adults from the same generation. A sample of the subsequent generation of adults was reared in the laboratory and tested from 7 to 90 days after adult eclosion to investigate whether changes in isozyme expression during the adult stage were associated with diapause, reproductive maturation, or overwintering. Frequencies of two isozymes of G6PDH (G6PDH-1 and G6PDH-2) and two isozymes of AO (AO-1 and AO-2) were different in weevils of different ages. The isozyme G6PDH-2 was present in 82–92% of newly eclosed weevils, whereas G6PDH-1 was present in 85–100% of both overwintered weevils and laboratory-reared weevils >60 days old. Newly eclosed weevils expressed only AO-2, whereas weevils just entering diapause expressed AO-1 and AO-2 simultaneously. Laboratory-reared weevils >60 days old expressed only AO-1, but AO-2 was present in 91% of overwintered males and 100% of overwintered females. Expression of an isozyme of LDH (LDH-H) was detected in all male weevils >60 days old, whether laboratory-reared or overwintered. No LDH-H was detected in young males or female weevils of any age. Thus, isozyme expression in adult alfalfa weevils can be influenced by developmental age (G6PDH, AO, and LDH), environmental conditions (AO), and sex (LDH). These results suggest that interpretation of electrophoretic phenotypes in insects be made with recognition of these potential effects.